JPH10195597A - Thin steel sheet excellent in joinability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet excellent in formability, free from fracture and deterioration in fatigue strength caused by molten metal brittleness at the time of brazing and welding and excellent in joinability. SOLUTION: This steel sheet has a compsn. contg., by weight, 0.003 to 0.01% C, 0.05 to 0.5% Mn, <=0.02% P, <=0.1% sol. Al, Ti: 48×(N/14) to 48× (N/14)+(S/32)}%, Nb: 93×(C/12) to 0.1%, 0.0005 to 0.003% B, <=0.01% N, <=0.05% Ni, and the balance Fe with inevitable impurities. Or, this is the one in which, in the above chemical compsn., the content of N is regulated to 0.004 to 0.01%, the content of Ni is regulated to <=0.05%, also, (Ni/Cu)>=1, and the content of Cu is regulated to <=0.05%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a mold having good moldability,
The present invention relates to a thin steel sheet which is excellent in joining property and is less likely to cause embrittlement or strength reduction due to the influence of heating when joining steel sheets by welding or brazing.

[0002]

2. Description of the Related Art In many cases, thin steel sheets used for automobiles and home electric appliances are assembled by brazing or welding using a solder alloy or brazing material after forming. At the time of brazing, the brazing material is heated to a temperature higher than the melting point and becomes a molten state, and then resolidifies. When a plated steel sheet is welded, the plated layer is once in a molten state and then re-solidified. When brazing or welding is performed in this way, there is a moment when a metal having a low melting point exists on the surface of the steel sheet.

In general, various residual stresses remain in products when pressed. When brazing material, Zn, Al, etc. come into contact with molten steel in a molten state at high temperature where residual stress exists, these metals penetrate into the grain boundaries of the steel plate and weaken the strength of the grain boundaries,
The grain boundary brittle fracture may occur, or the fatigue strength may be significantly reduced when subjected to repeated stress. This is commonly referred to as solder embrittlement or molten metal embrittlement.

For automobiles and home electric appliances, emphasis is placed on press formability, and extremely low carbon IF (Interstitial Fr).
ee) Steel plates are increasingly used. In the IF steel, all the C in the steel is fixed as carbide and the crystal grain boundaries are cleaned, so that the molten metal easily enters the grain boundaries. For this reason,
In an ultra-low carbon IF steel sheet or a plated steel sheet using the same as a base material, molten metal brittleness is likely to occur in a brazed portion or a weld heat affected zone.

[0005] Further, in a Zn-plated steel sheet or an Al-plated steel sheet using an ultra-low carbon IF steel as a base material, if a base material having a high Cu content is used, the steel in the vicinity of the weld tends to become brittle.
The reason for this is not clear, but Zn and Al in the plating layer and Cu in the steel sheet are melted and penetrate into the crystal grain boundaries near the molten portion, where Zn-Cu alloy and Al-Cu alloy are formed at the crystal grain boundaries. It is presumed that the crystal grain boundary is embrittled.
When welding an Al-plated steel sheet, embrittlement by Cu is more likely to occur than in a galvanized steel sheet. In the case of a Zn-plated steel sheet, Zn in the molten portion evaporates to some extent, but Al
It is presumed that the boiling point is higher than that of zinc, and Al is more likely to remain in the welded portion than Zn. This phenomenon is conspicuous in an ultra-low carbon IF steel sheet manufactured by fixing solid solution C as carbide. These problems are caused by I
This is a factor that hinders the use of F steel.

Various methods have been proposed for preventing brittleness of molten metal. JP-A-60-92453 discloses Ti-B
A cold-rolled steel sheet for brazing welding containing Cr is disclosed. Japanese Unexamined Patent Publication (Kokai) No. 63-243225 discloses that a very low C steel is made of Ti-B, Ti-B-Cr, Ti-Nb-B or T-B.
A method for producing a cold-rolled steel sheet containing i-Nb-B-Cr and having excellent brazing crack resistance is disclosed. JP-A-3-17
No. 3717 discloses a method for producing a copper-based cold-rolled steel sheet for brazing in which B is contained in a low C-Al killed steel. Japanese Unexamined Patent Publication (Kokai) No. 64-4456 discloses a copper-plated steel sheet having excellent weld cracking resistance using a base material containing B in a low C-Al killed steel and a method for producing the same. However, these methods alone are not enough to prevent the brittleness of molten metal.

The heat-affected zone near the weld is heated above the austenite zone. In brazing, the melting point of 4-6 brass, which is widely used because it is inexpensive and easy to apply, is 900 ° C.
Since it is before and after, at the time of brazing, the joint is heated to 950 ° C. or more, that is, to the austenite region. When the steel sheet is heated to a temperature higher than the austenite region, carbides in the steel sheet are dissolved, and thus austenite crystal grains may grow abnormally large. Ferrite crystal grains generated from coarse austenite crystal grains tend to have a large grain size, and the static strength and fatigue strength at that portion are likely to be reduced. In particular, this phenomenon is remarkable in an ultra low carbon IF steel having a clean grain boundary.

In the joining of steel sheets, it is desired to prevent both the brittleness of the molten metal and the decrease in the strength. However, none of the conventional methods is effective enough, and the joining members are required to secure the desired strength. In order to increase the thickness of the steel sheet or to remove the plating film of the welded portion and perform welding.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a thin steel sheet which is excellent in formability and has excellent weldability without causing brittleness of molten metal or reduction in bonding strength at a brazed portion or a welded portion. It is to be.

[0010]

SUMMARY OF THE INVENTION The gist of the present invention is a thin steel sheet having excellent bonding properties described in the following (1) and (2).

(1) C: 0.003-0.0% by weight
1%, Mn: 0.05 to 0.5%, P: 0.02% or less, sol. Al: 0.1% or less, Ti: 48 × (N /
14) to 48 × {(N / 14) + (S / 32)}%, N
b: 93 × (C / 12) -0.1%, B: 0.0005
0.003%, N: 0.01% or less, Ni: 0.05
% Or less and a chemical composition comprising the balance of Fe and unavoidable impurities, and having excellent bonding properties. Here, the symbol S represents the content (% by weight) of S in the steel sheet.

(2) C: 0.003-0.0% by weight
1%, Mn: 0.05 to 0.5%, P: 0.02% or less, sol. Al: 0.01 to 0.1%, Ti: 48 ×
(N / 14) to 48 × {(N / 14) + (S / 32)}
%, Nb: 93 × (C / 12) to 0.1%, B: 0.0
005 to 0.003%, N: 0.004 to 0.01%,
Ni: 0.05% or less, and (Ni / Cu) ≧ 1, having a chemical composition consisting of the balance Fe and inevitable impurities, and having a Cu content of 0.05% or less in inevitable impurities. Excellent thin steel plate. Here, the symbol S represents the content (% by weight) of S in the steel sheet.

The present invention has been completed by obtaining the following new knowledge regarding the change in the structure and strength of a steel sheet due to the heat history received during brazing and welding.

By precipitating NbC in the ultra-low carbon steel, abnormal growth of austenite crystal grains which is likely to occur when heated to the austenite region can be suppressed.

[0015] N generated when ultra-low carbon steel is converted to IF steel
Carbides such as bC and TiC are obstacles to the movement of crystal grain boundaries, and are effective means for preventing crystal grains from becoming coarse. However, at high temperatures such as in the austenitic region, any of the carbides re-dissolves in the steel.

When a steel sheet is heated, TiC hardly re-dissolves in the ferrite region, but re-dissolves at once in the austenite region. Austenitic crystal grains grow rapidly, on the contrary, because TiC, which has strongly suppressed crystal grain growth in the ferrite region, suddenly disappears. For this reason, when the steel sheet containing TiC is heated to the austenitic region, the steel may eventually have a coarser austenite grain structure than the steel not containing TiC.

On the other hand, when the steel sheet containing NbC is heated,
NbC gradually re-dissolves from the ferrite region, and the crystal grains gradually increase accordingly. For this reason, even after the steel sheet temperature reaches the austenite region and NbC completely re-dissolves, the crystal grains do not become so large. Accordingly, NbC rather than TiC is suitable as a carbide for preventing abnormal austenite grain growth.

If a predetermined amount or more of NbC is secured in a steel sheet and coexist with B, a hard acicular ferrite structure is generated by rapid cooling after brazing or welding. As a result, it is possible to prevent the strength of the brazed portion and the welded portion from decreasing. This is presumably because the once dissolved NbC is rapidly cooled before re-precipitation after welding, thereby producing a quenching effect.

[0019] When the ultra-low carbon IF steel, which is the base material of the plated steel sheet, contains an appropriate amount of Ni in accordance with the Cu content, embrittlement near the weld fusion zone is suppressed. Although the reason for this is not clear, a Cu—Ni alloy is formed when the molten portion solidifies, and a Zn—Cu alloy or an Al—C
It is presumed that the formation of the u alloy is suppressed.

In the IF steel, since the impurities in the steel are small, the crystal structure of the portion once melted at the time of welding tends to be coarse. For this reason, the static strength and fatigue strength of the weld may decrease. The crystal structure of the fusion zone is TiN
The fineness can be achieved by including. This is presumed to be due to the fact that TiN becomes a solidification nucleus and that TiN suppresses the movement of crystal grain boundaries.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting each factor and condition in implementing the present invention will be described below. The percentages of the chemical compositions described below mean% by weight.

C: Generally, from the viewpoint of moldability, the smaller the better, the better. However, in the present invention, it is positively added as a precipitate-forming element for preventing a decrease in the strength and fatigue strength of a welded product due to coarsening of the crystal grains in the heat-affected zone due to heating during welding. It is effective to increase the number of precipitates to prevent the strength from being reduced by heating. Therefore, the C content in steel is set to 0.003% or more. If the C content is too high, the amount of Nb required for fixing solid solution C increases, which impairs economic efficiency. On the other hand, when the amount of NbC precipitated is too large, the recrystallization temperature increases, and the formability of the steel sheet decreases.
Therefore, the upper limit of the C content is 0.01%. Desirably, 0.004 to 0.006% is contained.

Mn: S, which is an impurity in steel and causes hot brittleness, is first fixed in the present invention as TiS to render it harmless. However, as will be described later, it is necessary to limit the Ti content to a low level in order to avoid precipitation of TiC.
S may remain without forming TiS. Mn is
S not fixed as TiS is contained for fixing as MnS. Therefore, the lower limit of the Mn content is set to 0.05%. When Mn becomes excessive, all S in steel becomes M
Ti which becomes nS and becomes TiS forms TiC. TiC is not preferred because it may cause austenite crystal grains to become coarse and reduce the strength of the weld heat affected zone. To avoid this, the upper limit of the Mn content is set to 0.5%. P: contained in steel as inevitable impurities, but segregates at the grain boundaries in the heat-affected zone during welding or brazing, which may embrittle these portions. 0.02% or less.

Ti: added in order to improve the formability by fixing the total amount of N and the entire amount or a part of S in the steel as TiN or TiS. In the present invention, no TiC is formed. In order to fix the total amount of N, the lower limit of the Ti content is 48 ×
(N / 14)%. In order not to form TiC, T
The upper limit of the i content is 48 × ｛(N / 14) + (S / 3
2) Set to｝%.

Nb: Improves formability by fixing solid solution C as NbC, and uses NbC to suppress crystal grain growth in a high temperature range. Therefore, the lower limit of the Nb content is set to 93 × (C / 12)%. However, if the content exceeds 0.1%, the recrystallization temperature increases and the properties of the product deteriorate, so the upper limit is 0.1%.

B: It has the effect of segregating at the crystal grain boundaries of IF steel to prevent brittleness of molten metal during welding or brazing. In order to prevent the brittleness of molten metal, it is necessary to contain B by 0.0005% or more. On the other hand, if it is contained excessively, its prevention effect is saturated and the deep drawability deteriorates remarkably, so the upper limit of the content is made 0.003%. sol. Al: Although not an essential element, it has an action of deoxidizing steel, so it is desirable to use it as a deoxidizing agent. To ensure the deoxidizing effect, sol. Al is preferably added in an amount of 0.01% or more, and if sol.Al is excessively contained, economic efficiency is impaired, so the upper limit is set to 0.1%.

N: Although not an essential element, it combines with Ti to form a nitride, refines the crystal structure of the molten portion generated at the time of welding, and has the effect of improving the strength of the welded portion. May be. In order to sufficiently obtain the effect of refining the crystal structure, N is preferably contained at 0.004% or more. If N is contained in an amount exceeding 0.01%, the amount of Ti added becomes large, resulting in lack of economy, and the precipitation amount of TiN becomes large, thereby impairing the formability of the steel sheet. Therefore, the upper limit of N is set to 0.01%.

Ni: has an effect of suppressing embrittlement in the vicinity of a molten portion, which tends to occur when welding a steel sheet plated with Zn or Al to a base material containing a large amount of Cu as an unavoidable impurity. For this reason, Ni can be contained as needed.
In order to obtain this effect, it is preferable to contain Ni at least one time the Cu content. Even if it is contained excessively, the effect of suppressing embrittlement is saturated and workability is impaired, so the upper limit is made 0.05%.

Other than the above, Fe and inevitable impurities are included. Among the inevitable impurities, Cu and S are preferably limited as follows.

Cu: an unavoidable impurity that is mixed in from the raw material to be dissolved. When Zn and Al coexist in a high temperature state, they enter the crystal grain boundaries together with Zn and Al at the time of welding and embrittle the vicinity of the molten portion. In order to effectively suppress the embrittlement of the welded portion by adjusting the Ni content, the Cu content is set to 0.05% or less.

S: contained in steel as an unavoidable impurity, but may cause red hot embrittlement during hot rolling of steel, and impair the formability of steel sheet, so that 0.01%
It is better to do the following.

Although the method for producing a thin steel sheet of the present invention is not specified, for example, it is melted by a commonly used method such as a converter or an electric furnace and, if necessary, subjected to a vacuum degassing treatment or the like. A steel having the above chemical composition, a slab is formed by a method such as a method of bulk rolling into a steel ingot or a continuous casting method, and hot rolling is performed.
Pickling is performed as necessary to obtain a hot-rolled steel sheet. In addition, cold rolling, annealing, temper rolling and the like may be performed to form a cold-rolled steel sheet, or plating may be performed on the surface of the hot-rolled steel sheet or the cold-rolled steel sheet by an ordinary method. As the metal to be plated, a metal or an alloy having a lower melting point than Fe can be applied, for example, Zn,
Examples include Al, Pb, Sn, and the like, and Zn-Fe, Zn-Al, and other alloys. Although the plating method is arbitrary, for example, an electroplating method, a hot-dip plating method, a vapor deposition plating method, or the like can be applied.

When used as a hot-rolled steel sheet, the winding temperature in the hot-rolling step is preferably set to 550 ° C. or higher in order to improve the formability of the steel sheet. When used as a cold-rolled steel sheet, it is economically preferable to perform the annealing by a continuous annealing method. When performing continuous annealing, after cooling at an annealing temperature, cooling to 650 ° C.
It is preferable to gradually cool at a cooling rate of not more than ° C / sec. These treatments promote the segregation of B into ferrite grain boundaries,
Melt metal brittleness is further improved.

[0034]

【Example】

(Example 1) A steel having a chemical composition shown in Table 1 was melted by using an experimental vacuum melting furnace, and a steel sheet having a thickness of 25 was formed by hot forging.
mm and a width of 210 mm.

[0035]

[Table 1]

These were heated in an electric furnace at 1250 ° C. for 1 hour, and then rolled to a thickness of 5 mm in a temperature range of 1150 ° C. to 930 ° C. by three-pass rolling using an experimental hot rolling mill. Immediately after the rolling, the material is cooled to a temperature of 450 to 800 ° C. by forced air cooling or water spray cooling, placed in an electric furnace maintained at each temperature, and maintained for 1 hour.
In some cases, the furnace was cooled to room temperature. The front and back surfaces of the obtained steel sheet were ground to a thickness of 3.2 mm, and this was cold-rolled to a thickness of 0.8 mm.
A cold-rolled plate having a width of 210 mm and a width of 210 mm was obtained. These cold rolled sheets were heated in an infrared heating furnace to 820 ° C. at a heating rate of 10 ° C./sec, held for 40 seconds, gradually cooled to 650 ° C. at a cooling rate of 3 ° C./sec, and thereafter cooled to 50 ° C. Cooled to room temperature at a cooling rate of ° C / sec. After annealing, temper rolling at an elongation of 0.8% was performed. The formability of the obtained cold-rolled steel sheet was measured according to JIS Z
Evaluation was made based on the tensile properties of a No. 5 tensile test piece specified in 2201.

The brittle metal resistance was evaluated by the method shown in FIG. A tensile load by the weight 2 is applied to the steel sheet sample 1 having a parallel portion having a width of 25 mm and a length of 60 mm via a holder 3 and a wire 4 provided at both ends. In this state, a flux (F10S manufactured by Nice Co., Ltd.) is placed at the center of the upper surface of the steel plate sample 1, and heated to 950 ° C. at a heating rate of 20 ° C./sec by a gas burner (not shown) on the lower surface of the steel plate sample. The brazing material 6 is BCuZ specified in JISZ3262.
Use n-2 (4-6 brass) and place it on the flux. Although the temperature of the steel sheet sample once decreases due to the melting of the brazing material, the temperature is returned to 950 ° C. again, and the load is maintained for 30 seconds. The test was performed by changing the mass of the weight 2, the lower limit load for brittle fracture in the above 30 seconds was obtained, and the stress on the initial cross section was calculated and defined as "critical brittle stress". The temperature of the sample was measured with a thermocouple (not shown) attached to the sample.

The strength reduction of a steel sheet caused by heating to a high temperature during brazing or welding is based on the fact that the tensile fracture stress at room temperature of a heat-treated steel sheet simulating these heat histories is determined by JI.
It was determined and evaluated using a No. 5 tensile test piece specified in SZ2201. Hereinafter, this stress is referred to as “rupture stress after heat treatment”. In the above heat treatment, a steel sheet sample is heated to 950 ° C. at a heating rate of 20 ° C./sec in an electric furnace, held for 30 seconds, air-cooled to 500 ° C., and then water-cooled to room temperature.

Table 2 shows the measurement results of the tensile properties, critical embrittlement stress, and rupture stress after heat treatment of the steel sheet sample.

[0040]

[Table 2]

As shown in Table 2, all of the steel plates prototyped from steels 1 to 5 satisfying the chemical composition of the present invention have an embrittlement critical stress of 18 MPa or more and a rupture stress after heat treatment of 30.
It has excellent brazeability of 0 MPa or more. These steel sheets exhibit tensile properties with an r value of 1.7 or more and a total elongation of 45% or more, and have good press formability. On the other hand, the steel sheet of Steel 6 having a C content less than the range specified in the present invention has a reduced fracture stress after heat treatment. The steel sheet 7 containing excessive amounts of C and Nb and the steel sheet 9 containing excessive amounts of B all have good embrittlement critical stress, but exhibit formability due to excessive precipitation of NbC or excessive solid solution of B. The characteristics have deteriorated. The steel sheet 8 of steel 8 lacking B has a low critical stress for embrittlement, and the steel sheet 10 of steel 10 containing excess Ti is TiC.
Was excessively precipitated, and the crystal grains became coarse during heating at a high temperature, so that the breaking stress after the heat treatment was reduced. Steel 11 has a high P content and is inferior in embrittlement critical stress. Steel 12 has an excessive N content, and thus has poor moldability.

Example 2 Using an experimental vacuum melting furnace,
Steel having the chemical composition shown in Table 3 was melted and hot forged into a 25 mm thick, 210 mm wide experimental slab.

[0043]

[Table 3]

These were subjected to hot rolling, cold rolling, annealing and temper rolling under the same conditions as described in Example 1 to obtain a cold-rolled steel sheet having a thickness of 0.8 mm and a width of 210 mm. These cold-rolled steel sheets were subjected to Zn plating with an adhesion amount per side of 30 g / m ² on both sides by an electroplating simulator. JIS Z 220 from the obtained Zn-plated steel sheet
A No. 5 tensile test piece specified in 1 was collected and subjected to a tensile test. Further, the embrittlement critical stress and the rupture stress after the heat treatment were determined in the same manner as described in Example 1. Further, two test pieces having a width of 50 mm obtained from the respective plated steel sheets were overlapped, and spot welding was performed using a dome-shaped electrode having a diameter of 6 mm under the conditions of a current supply time of 10 cycles, a current of 8 kA, and a pressure of 200 kgf. After the welding, a tensile load of 0 to max. Tested in pulsating method for adding repeatedly between, was determined maximum load is not broken at ¹⁰⁷ iterations (tensile shear fatigue strength).
Table 4 shows the results of these various performance measurements.

[0045]

[Table 4]

Steel A having the chemical composition defined by the present invention,
B, C, D, E and F all have good formability, and also have good brittle critical stress and good breaking stress after heat treatment. Further, the steels A, B, C and D whose N and Ni contents are in the preferable ranges defined by the present invention have good shear tensile fatigue strength of the spot welded portion of 1290 N or more. On the other hand, in the steel E in which the N content is lower than the preferable range, the crystal structure of the melted portion becomes coarse, so that the fatigue strength is slightly inferior to the steel A or the like, and the Ni / Cu ratio is lower than the preferable range. In the case of steel F, the fatigue strength of the welded portion is lower than that of steel A or the like. Steel G whose Cu content is outside the upper limit specified in the present invention
In this case, the fatigue strength is reduced despite the excessive addition of Ni.

[0047]

The thin steel sheet of the present invention has good formability, excellent brittle metal resistance, excellent strength at the brazed portion, and excellent fatigue strength at the welded portion.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an outline of a method for measuring a critical stress for embrittlement resistance to molten metal.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate sample 2 Weight 3 Steel plate sample holder 4 Wire for load application 5 Fixed wall 6 Brazing filler metal

Claims (2)

[Claims] (1) C: 0.003 to 0.01% by weight,
Mn: 0.05-0.5%, P: 0.02% or less, so
l. Al: 0.1% or less, Ti: 48 × (N / 14) ~
48 × {(N / 14) + (S / 32)}%, Nb: 93
× (C / 12) to 0.1%, B: 0.0005 to 0.0
03%, N: 0.01% or less, Ni: 0.05% or less,
A thin steel sheet having a chemical composition consisting of a balance of Fe and unavoidable impurities and having excellent bonding properties. Here, the symbol S is S in the steel sheet.
(% By weight). 2. C: 0.003 to 0.01% by weight,
Mn: 0.05-0.5%, P: 0.02% or less, so
l. Al: 0.01 to 0.1%, Ti: 48 × (N / 1
4) -48 × {(N / 14) + (S / 32)}%, N
b: 93 × (C / 12) -0.1%, B: 0.0005
-0.003%, N: 0.004-0.01%, Ni:
0.05% or less, (Ni / Cu) ≧ 1, has a chemical composition consisting of the balance of Fe and inevitable impurities, and has excellent bondability in which the Cu content in the inevitable impurities is 0.05% or less. Sheet steel. Here, the symbol S represents the content (% by weight) of S in the steel sheet.